1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a LCD device having an improved aperture ratio.
2. Description of the Related Art
Presently, LCD devices are developed as next generation display devices because of their light weight, thin profile, and low power consumption characteristics. In general, an LCD device is a non-emissive display device that displays images using a refractive index difference utilizing optical anisotropy properties of liquid crystal material that is interposed between an array (TFT) substrate and a color filter (C/F) substrate. Among the various type of LCD devices commonly used, active matrix LCD (AM-LCD) devices have been developed because of their high resolution and superiority in displaying moving images. The AM-LCD device includes a thin film transistor (TFT) per each pixel region as a switching device, a first electrode for ON/OFF, and a second electrode used for a common electrode.
FIG. 1 is a perspective view of an LCD device according to the related art. In FIG. 1, first and second substrates 10 and 30 are arranged to face each other with a liquid crystal material layer 50 interposed therebetween. On an inner surface of the first substrate 10, a color filter (C/F) layer 12 and a common electrode 16, which functions as one electrode for applying an electric field to the liquid crystal layer 50, are subsequently formed. The color filter layer 12 includes a color filter for passing only light of a specific wavelength, and a black matrix (not shown) that is disposed at a boundary of the color filter and shields light of a region in which alignment of the liquid crystal material is uncontrollable. On an inner surface of the second substrate 30, a plurality of gate lines 32 and a plurality of data lines 34 are formed in a matrix array. A TFT “T”, which functions as a switching device, is disposed at a region where each gate line 32 and data line 34 crosses, and a pixel electrode 46 that is connected to the TFT “T” is disposed at a pixel region “P” defined by the region where the gate and data lines 32 and 34 cross. First and second polarizing plates 52 and 54, which transmit only light parallel to a polarizing axis, are disposed on an outer surface of the first and second substrates 10 and 30, respectively. An additional light source such as a backlight, for example, is disposed over the polarizing plate 54.
An aperture ratio of an LCD device is defined as a ratio of an area capable of displaying information to a total display area. As the aperture ratio increases, an area of a pixel electrode increases.
FIG. 2A is a cross-sectional view of an LCD device showing a TFT portion according to the related art. In FIG. 2A, first and second substrates 1 and 2 are arranged to oppose and face each other with a space formed therebetween. A gate electrode 31 is formed on an inner surface of the second substrate 2, and a gate insulating layer 33 is formed on the gate electrode 31. A semiconductor layer 36, formed by subsequently depositing an active layer 36a and an ohmic contact layer 36b, is formed on the gate insulating layer 33 over a gate electrode 31. Source and drain electrodes 38 and 40 spaced apart from each other are formed on the semiconductor layer 36 with a space between the source and drain electrodes 38 and 40, thereby forming a channel “ch” exposing the active layer 36a. The gate electrode 31, the semiconductor layer 36, and the source and drain electrodes 38 and 40 comprise a TFT “T”. Moreover, a passivation layer 42 is formed on the TFT “T,” and a pixel electrode 46 is formed on the passivation layer 42. The passivation layer 42 has a drain contact hole 44 that exposes a surface region of the drain electrode 40, and the pixel electrode 46 contacts the surface region of the drain electrode 40 through the drain contact hole 44 with the pixel region “P” (of FIG. 1). A second orientation film 48 is formed on the passivation layer 42 and the pixel electrode 46 to induce an alignment of a liquid crystal material layer 50. In addition, a color filter layer 13 and a black matrix 14 are formed on an inner surface of the first substrate 1 to overlap each other. A common electrode 16 and a first orientation film 18 are subsequently formed on the color filter layer 13 and the black matrix 14. Amorphous silicon (a-Si), which can be applied during a low temperature process, is mainly used as the semiconductor layer 36 of the second substrate 30. Since a photo current is generated in an a-Si TFT by light, the black matrix 14 is formed to cover the TFT “T”.
FIG. 2B is a cross-sectional view of an LCD device showing a data line portion according to the related art. In FIG. 2B, a data line 34 and a pixel electrode 46 are formed to be spaced apart from one another on an inner surface of the second substrate 2. A black matrix having first and second regions “d1” and “d2” that correspond to a distance between adjacent pixel electrodes 46 and an attachment margin (about ±5 μm) are disposed on the inner surface of the first substrate 1, respectively. The second regions “d2” of the black matrix 14 extend to overlap the pixel electrode 46 and surround a boundary (not shown) of the pixel electrode 46, thereby preventing light leakage at the first region “d1” when the first and second substrates 1 and 2 are misaligned. Since the data line 34 is parallel to a long axis of the pixel electrode 46, the second region “d2” over the data line 34 has a great influence on an aperture ratio of the LCD device.
FIG. 3 is a plan view of an LCD device according to the related art of a light-leaking region in a two-domain structure. An LCD device according to the related art has a mono-domain structure in which an alignment of a liquid crystal material is kept uniform through an entire substrate. Presently, an LCD device of a multi-domain structure is being developed in which one pixel region is divided by an electrical or intrinsic property of the liquid crystal material and the liquid crystal layers of the divided regions are aligned differently. In FIG. 3, a twisted nematic (TN) liquid crystal material of a normally white mode is used, and rubbing directions of first and second substrates are 45° and 135°, respectively. When a voltage is applied in an LCD device of a normally black mode, an additional shield for light leakage is not necessary, since the light-leaking region is a white line and the other region is white. When a voltage is not applied in the LCD device of a normally black mode, the light leakage region does not influence display quality, since the liquid crystal material is nearly parallel to the substrate. Accordingly, an LCD of a normally white mode is adopted in FIG. 3.
In FIG. 3, first light leakage regions “A” are generated long a direction parallel to a long axis of a pixel electrode, and a second light leakage region “B” is generated along the direction parallel to a short axis of the pixel electrode. The first light leakage regions “A” are located at opposite positions according to the domain, and the second light leakage region “B” is located at a border between the domains. When a voltage is applied, a reverse tilt domain whose alignment is reverse to that of liquid crystal is formed near the data line by a lateral electric field between the data line and the pixel electrode. Accordingly, liquid crystal material along a border between reverse tilt and normal domains are not controlled when a voltage is applied, since an alignment of the reverse tilt domain is reversed to an alignment of the liquid crystal material formed near the data line due to a lateral electric field between the data line and the pixel electrode. Thus, the first light leakage regions “A” are generated. Moreover, in the two-domain structure, since the liquid crystal material of the first and second domains are aligned along different directions from each other, a disclination is generated at a border between the first and second domains. Thus, the second light leakage region “B” is generated. Accordingly, in a multi-domain structure, a black matrix pattern should include light leakage regions by a disclination between domains and a lateral electric field between a data line and a pixel electrode.
FIG. 4 is a plan view of an LCD device according to the related art of a two-domain structure showing a black matrix pattern. In FIG. 4, a plurality of data lines 60 are spaced apart from each other, and a pixel electrode 62 is disposed between adjacent data lines 60. A black matrix pattern includes a first black matrix 64 corresponding to the data line 60 and a second black matrix 66 having first, second, and third sub-black matrix regions “A1”, “B1” and “C1,” thereby the first and second black matrices 64 and 66 are interconnected. The first and second sub-black matrices “A1” and “B1” overlapping the pixel electrode 62 correspond to the first and second light leakage regions A and B (in FIG. 3). The third sub-black matrix “C1” results from forming the black matrix pattern as a stripe, thereby reducing an aperture ratio without providing a shielding effect of the light leakage. Accordingly, since the black matrix pattern of a multi-domain structure includes sub-black matrices to shield light leakage at borders between the domains, the aperture ratio of a multi-domain structure additionally decreases in contrast with that of mono-domain structure.